UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

(Rule 14a-101)

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

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☐ Preliminary Proxy Statement

☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

☐ Definitive Proxy Statement

☒ Definitive Additional Materials

☐ Soliciting material Pursuant to §240.14a-12

Automatic Data Processing, Inc.

(Name of Registrant as Specified In Its Charter)

William A. Ackman

Veronica M. Hagen

V. Paul Unruh

Pershing Square Capital Management, L.P.

PS Management GP, LLC

Pershing Square, L.P.

Pershing Square II, L.P.

Pershing Square International, Ltd.

Pershing Square Holdings, Ltd.

Pershing Square VI Master, L.P.

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

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On September 5, 2017, Pershing Square Capital Management, L.P. and certain affiliates issued the following press release:

Pershing Square Files Definitive Proxy Materials for ADP 2017 Annual Meeting

Urges ADP Stockholders to VOTE the GOLD Proxy Card to Send a Message to ADP’s Board to Change the Status Quo

New York, NY – September 5, 2017 //- Pershing Square Capital Management, L.P. (“Pershing Square”), which manages funds that beneficially own approximately 8.3% of the outstanding shares of ADP (NASDAQ: ADP), today filed a definitive proxy statement with the Securities and Exchange Commission for the election of William A. Ackman, Veronica M. Hagen and V. Paul Unruh (together, “the Nominees for ADP’s Transformation”) to the ADP Board of Directors (the “Board”) at the upcoming annual meeting (the “2017 Annual Meeting”).

A copy of the proxy statement can be accessed at www.ADPascending.com.

“We look forward to the 2017 Annual Meeting where stockholders will be given the opportunity to elect Pershing Square’s independent directors who bring a long-term shareholder orientation, fresh perspectives, and relevant expertise to accelerate shareholder value creation at ADP,” said Bill Ackman, CEO of Pershing Square.

You can vote by Internet, telephone or by signing and dating the GOLD Proxy Card or Voting Instruction Form and mailing it in the postage paid envelope provided. We urge you NOT to vote using any white proxy card or voting instruction forms you receive from ADP. Please discard the white proxy card.

If you have any questions about how to vote your shares, please contact our proxy solicitor, D.F. King & Co., Inc., at (866) 342-1635.

About Pershing Square Capital Management, L.P.

Pershing Square Capital Management, L.P., based in New York City, is a SEC-registered investment advisor to investment funds.

Media Contact:

Pershing Square

Fran McGill

212 909 2455, McGill@persq.com

Pershing Square Capital Management, L.P. (“Pershing Square”) and certain of its affiliated funds have filed with the Securities and Exchange Commission (the “SEC”) a definitive proxy statement and accompanying GOLD proxy card to be used to solicit proxies in connection with the upcoming annual meeting of stockholders (the “Annual Meeting”) of Automatic Data Processing, Inc. (the “Company”) and the election of a slate of director nominees at the Annual Meeting (the “Solicitation”). Stockholders are advised to read the proxy statement and any other documents related to the Solicitation because they contain important information, including information relating to the participants in the Solicitation. These materials and other materials filed by Pershing Square with the SEC in connection with the Solicitation are available at no charge on the SEC’s website at http://www.sec.gov. The definitive proxy statement and other relevant documents filed by Pershing Square with the SEC are also available, without charge, by directing a request to Pershing Square’s proxy solicitor, D.F. King & Co., Inc., 48 Wall Street, 22nd Floor, New York, New York 10005 (Call Collect: (212) 269-5550; Call Toll Free: (866) 342-1635) or email: ADP@dfking.com.

William A. Ackman, Veronica M. Hagen, V. Paul Unruh, Pershing Square, PS Management GP, LLC (“PS Management”), Pershing Square, L.P., Pershing Square II, L.P., Pershing Square International, Ltd., Pershing Square Holdings, Ltd. and Pershing Square VI Master, L.P. may be deemed “participants” under SEC rules in the Solicitation. William A. Ackman, Pershing Square and PS Management may be deemed to beneficially own the equity securities of the Company described in Pershing Square’s statement on Schedule 13D initially filed with the SEC on August 7, 2017 (the “Schedule 13D”), as it may be amended from time to time. Except as described in the Schedule 13D or the definitive proxy statement, none of the individuals listed above has a direct or indirect interest, by security holdings or otherwise, in the Company or the matters to be acted upon, if any, in connection with the Annual Meeting.